1. Field of the Invention
This invention relates generally to biologically catalyzed processes which can occur in man-made environments or natural ecosystems and, more particularly, to a method for monitoring and controlling the condition of such processes as anaerobic digestion.
2. Background Art
Anaerobic digestion is a biologically catalyzed process in which organic matter is decomposed, in the absence of oxygen, primarily to the gaseous end products of carbon dioxide and methane. Naturally occurring habitats for the unique consortium of microbes that carries out this sequence of fermentative reactions include marshes, rice paddies, benthic deposits and ruminants (such as cattle, sheep and buffalo). Anaerobic digestion is well known to sanitary engineers and has been used for sludge treatment at wastewater treatment plants for over 100 years. The primary purpose of the process has been for sludge digestion to achieve waste stabilization and solids reduction. Over the recent past it has also been applied for the treatment of both liquid industrial and municipal wastewaters.
Due to the inherent slow growth rate of the methane producing bacteria, conventional anaerobic digesters are sensitive to changes in hydraulic loading, organic loading and temperature, and recover slowly once they are upset. Advances in reactor technology (i.e., fixed film systems) have effectively eliminated the problem of hydraulic overloading for industrial wastewater treatment. Upsets as a consequence of organic overloading or inadvertent toxicant induced inhibition still are potential problem areas. The last remaining hurdle to widespread application of anaerobic treatment to industrial wastewaters is improved process reliability.
Unfortunately, despite the great strides that have been made toward increasing the fundamental understanding of how anaerobic systems function during the past fifteen years, little of this information has been integrated into process control strategies and testing procedures. The same analyses and process control techniques developed almost two decades ago are still utilized today.
Ideal indicators should be capable of measuring the progress of sludge digestion and signal impending upsets before they occur. Several of the more commonly used indicators include: (1) volatile acids to alkalinity ratio; (2) gas production rates and gas composition; (3) pH; and (4) volatile solids reduction (digester efficiency). None of these can accurately indicate the condition of the digestion process singly. Several of these indicators are usually considered together to properly control the process.
Although these indicators are useful for monitoring gradual changes, they do not directly reflect the current metabolic status of the active organisms in the digester. These common indicators are useful for detecting process upsets once they are underway. In many instances this may be adequate to avoid system failure for slow to develop difficulties such as a gradual organic or hydraulic overload.
More rapid monitoring techniques are needed, however, to avoid significant process deterioration and possible failure, especially for systems with relatively short hydraulic residence times (HRT) such as the various fixed-film reactor configurations and anaerobic contact processes.
There have been efforts to develop better control strategies--mostly relating digester performance with parameters associated with metabolic activity such as enzyme activity levels, and specific electron carriers. These newer techniques include acid and alkaline phosphotase activity, dehydrogenase activity, adenosin triphosphate activity and factor 420 levels.
All of the above attempts have met with limited success. They require rather sophisticated and time consuming wet chemical techniques that do not lend themselves to on-line measurement. Conclusive evidence that any of these approaches, even if practical assay techniques suitable for on-line measurement can be developed, can accurately function as acceptable indicators has not been demonstrated.
Analyzing parameters in the gaseous headspace offers several distinct advantages over liquid or slurry phase monitoring. First, process instrumentation in the field of gas chromatography and related fields has advanced much more quickly than its liquid counterpart. Second, the gas phase of an anaerobic treatment system of any type, is considerably more amenable to on-line monitoring than the liquid phase. Sampling conditions are much less severe with respect to the potential for chemical or physical fouling.
Analysis of the gas phase of anaerobic systems is, in fact, a vital part of current process control strategies. Monitoring has been, however, confined to the primary gaseous components, methane and carbon dioxide. No insight into the current metabolic status of the organisms involved is obtained from monitoring these two gaseous products. There has been, however, at least one anaerobic process control system proposed based on, in part, measuring, methane. Ochiai (U.S. Pat. No. 4,349,435) proposed controlling anaerobic treatment systems based on performing mass balances of oxygen demand (OD) in the influent, effluent and the equivalent OD leaving as methane gas. The admitted deficiency of not being able to reliably measure the oxygen demand of liquid samples severely limits this approach.
The use of monitoring trace gases that are somehow linked to the metabolic status of the major biochemical process occurring has been proposed by Saito et al. (U.S. Pat. No. 4,437,992), who describe a control system for activated sludge based upon measuring, among other parameters, N.sub.2 O, a reported metabolic intermediate in nitrogen conversion. However, the exact nature of the relationship of N.sub.2 O to the process is unclear from the teaching of Saito et al. and the patent is specific for the aerobic activated sludge process.
It has been suggested that there is the potential to obtain some measure of the metabolic status of anaerobic systems by monitoring the concentration of an intermediate product, hydrogen, which is present at trace levels in well-functioning treatment systems. This is based upon the fact that hydrogen is the direct precursor of approximately 30 percent of the methane produced in anaerobic digesters and industrial waste systems.
The patent of Friedman et al. (U.S. Pat. No. 4,690,755) makes mention of using hydrogen as a control parameter. Hydrogen, in this context, represents the product of one biochemical reaction and is the substrate of a sequential reaction that combines hydrogen with carbon dioxide to form methane. Accordingly, hydrogen is, in this case, an intermediate product and not a metabolic intermediate within a biochemical reaction as in N.sub.2 O.
It is recognized that approximately 70 percent of the methane formed in these anaerobic treatment systems is formed from conversion of acetate to methane and carbon dioxide via the reaction below: EQU CH.sub.3 COO.sup.- +H.sup.+ .fwdarw.CH.sub.4 +CO.sub.2 (1)
Monitoring strategies based on measuring hydrogen gas only, do not yield any information concerning this important biochemical reaction. This represents a severe limitation since acetate conversion to methane is believed to be the most sensitive and rate limiting step for methanogenesis.
A need thus exists for a new process monitoring approach capable of providing on-line, a rapid, reliable and accurate indication of the metabolic status of anaerobic digestion and other biologically catalyzed processes.